Lesson 1 of 6 · Binaries, eclipses & variability
Read the changing light of stars
10 minute lesson · 3 checks along the way
Your goal for this lessonUse a repeating light curve and complementary observations to distinguish an orbital cycle from a brightness change.
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Two stars can share an orbit
A is a gravitationally bound pair whose members orbit their common center of mass. Two points appearing close together in the sky need not form a binary: they may lie at very different distances. Establishing a physical pair requires evidence such as compatible distances and orbital motion. Conversely, two stars too close for a telescope to resolve can still reveal their partnership through changing light or spectra.
Variability describes a change in observed brightness, not one particular cause. A star can pulsate, rotating spots can alter its visible surface, or orbiting material can block some light. In an , the viewing geometry lets one star pass in front of the other. Many binaries never eclipse from our viewpoint. A quiet therefore cannot establish that a star has no companion.
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A photograph starts the measurement
Kepler’s first-light image contains many stellar sources spread across its segmented detector field. It records an exposure, not the histories of all those stars. Repeated calibrated measurements turn a source’s brightness into a light curve: a graph of received light against time. Background light, neighboring stars, detector behavior, and gaps in the observations must be considered before assigning a change to the target.
An eclipse removes some of the combined light reaching us while the stars continue emitting. The depth depends on the blocked area, the surfaces’ brightness, and viewing geometry. A deeper dip does not automatically identify the more massive star, and a brightness decrease alone does not establish a planet. Spectra, repeated timing, and a model of both luminous bodies help separate these possibilities.
Kepler’s first-light image combines the full photometer focal plane in a 60-second exposure before science operations. The panel boundaries belong to the detector arrangement; this displayed field is not a time series of stellar variability.
Look closely
Choose one bright point and one dark panel gap. Can this single exposure establish a four-day eclipse period?
See what to notice
No. The point locates a measured source and the gap reflects detector coverage. A period requires repeated calibrated brightness measurements at known times, with source blending and observing gaps evaluated.
Work through a decision
Count the repeating pattern, not every dip
Consider a hypothetical, circular eclipsing binary with 1.00 outside eclipse. Deep minima reach 0.70 at days 0, 4, and 8; shallow minima reach 0.90 at days 2 and 6. The complete deep–shallow sequence repeats every four days. The two-day interval separates different conjunctions, not a full return to the same arrangement. These invented values illustrate timing and are not observations of a Kepler target.
The deep event removes 1.00−0.70 = 0.30, or 30 percent, of the combined baseline light. That is a flux fraction, not directly a radius or mass fraction. Folding these measurements on a two-day period would mix deep and shallow events at one phase. Folding on four days keeps them distinct. Real equal-depth eclipses, irregular sampling, or eccentric orbits can make the inference less straightforward.
Follow a complete eclipse cycle
Original instructional diagram with invented data. A full deep–shallow pattern repeats in four days; the two-day interval joins unlike conjunctions. Straight segments simplify the eclipse shape.
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Add the measurement that tests the explanation
If the proposed pair has two detectable spectra, alternating can trace the components approaching and receding. Orbital timing and velocity amplitudes, combined with inclination constraints, can yield masses. A resolved orbit also needs its physical scale, usually requiring distance. Kepler’s orbital law alone cannot turn an angular separation and an uncertain period into uniquely determined stellar masses.
Plan a follow-up for the toy curve: sample both kinds of dip and obtain spectra at several . A companion model predicts a coordinated relationship between the eclipse timing and the velocity changes. A rotating spot or pulsating single star requires a different explanation. The aim is not merely to find a periodic pattern, but to test whether one physical account explains all the measurements and their uncertainties.
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